The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with imaging as a guide for other, possibly invasive, procedures and will be described with particular reference thereto. It is to be appreciated, however, that the present invention may also find application in other procedures in which it is advantageous to determine the location of portions of the patient's anatomy, diagnostic or surgical instruments, and the like, relative to reconstructed images and each other, and is not limited to the aforementioned application.
In magnetic resonance imaging, a substantially uniform main magnetic field is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a patient being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the main magnetic field by transmitting radio frequency excitation signals into the examination region. Specifically, radio frequency pulses transmitted via a radio frequency coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The radio frequency coil assembly is tuned to the resonance frequency of the dipoles to be imaged in the main magnetic field. For example, for protons in a 0.23 T field, the coil assembly is designed for optimal performance at 9.8 MHZ. The precessing magnetic moment, in turn, generates a corresponding radio frequency magnetic signal as it relaxes and returns to its former state of alignment with the main magnetic field. The radio frequency magnetic resonance signal is received by the radio frequency coil assembly which is again tuned to the resonance signal. From the received signals, an image representation is reconstructed for display on a human viewable display. Spatial position is encoded with magnetic field pulses that alter resonance frequency in accordance with spatial position. With a 9.8 MHZ nominal resonance frequency, the spatial encoding pulses typically shift the resonance frequency over about 200 kHz.
Previously, imageable fiducials have been used to correlate identified points on the surface of the patient with corresponding points in an image. Typical fiducials are hollow beads filled with a proton solution, such as copper sulfate (CuSO4) in an aqueous solution. In magnetic resonance imaging, the fiducials act similarly to dipoles in the subject. When subjected to the B0 main magnetic field, dipoles within the aqueous solution line up, and are tipped, refocused, and otherwise perturbed by the RF pulses. The fiducials show in a final image as bright dots and are used as points of reference for navigating an image. A problem with aqueous fiducials of this sort is that the resonance frequencies of the water in the fiducial and the water in the body are too close together, i.e., substantially the same. The fiducial marks tend to strip the imaged volume of magnetization and can saturate the spins in adjacent tissues.
Local transmit/receive coils have been used to isolate the fiducials. Typically, each fiducial has its own associated coil with a set of leads. The multiplicity of lead wires increase complexity within the imaging region and increase risk of RF burns.
Electron spin resonance (ESR) fiducials have also been used. These fiducials function similarly to the proton fiducials, except that their resonant frequencies are substantially higher. This type of system requires extra hardware. Specifically, a second set of transmitting and receiving coils are added for the microwave signals along with supporting transmitters and receivers. This increases complexity and cost.
Optical systems have also been used to track optical fiducials, as well as the surface of a subject directly. Typically, a number of cameras continually track the position of passive reflectors or active light emitters located on the subject and associated instruments. Images from the multiple cameras are used to triangulate positions of the light sources. Optical navigation systems are complex and expensive, requiring precise cameras. Optical systems must be preregistered to coincide with the physical structure of the scanner and the resultant image. Additionally, the cameras must have a line of sight to the optical emitters in order to detect the emitters, which in some cases is difficult and cumbersome.
Mechanical navigators have also been used to probe the position of a subject. Such a system may include a robot arm with instrumented joints or similar devices. It is difficult to manufacture such a system out of completely non-magnetic materials so as not to interfere with the main magnetic field. In some instances, the arm obstructs access to the subject.
The present invention contemplates a new and improved fiduciary detection method and apparatus which overcomes the above referenced disadvantages and others.